Liquid crystal display and method for driving the same

ABSTRACT

A liquid crystal display includes a plurality of gate lines and a plurality of data lines crossing over the gate lines while being electrically insulated from the gate lines. Pixels are placed at the cross regions of the gate and the data lines arranged in a matrix form. Each pixel has a switching circuit connected to the gate and the data lines. Data voltages are fed to the pixels such that the polarity of the pixels is inverted per a pixel group of two or more pixel rows. Gate voltages are applied to the neighboring first and second pixel groups such that the gate voltage applied to the pixel row of the first pixel group close to the second pixel group differs from the gate voltage applied to the pixel row of the first pixel group distant to the second pixel group.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display and a methodof driving the same and, more particularly, to a liquid crystal displaywhich is driven by an inversion driving method.

2. Description of the Related Art

Generally, a liquid crystal display structure includes a liquid crystallayer having a dielectric anisotropy sandwiched between two opposingsubstrates. An electric field is applied to the liquid crystal layerwith various in strength, thereby controlling light transmission anddisplaying the desired picture image.

A plurality of pixel electrodes are arranged on one of the substrates ina matrix form, and counter electrodes are arranged on the othersubstrate such that they correspond to the pixel electrodes. Each of theelectrode pair operates with the interposed liquid crystal therebyforming a liquid crystal cell, and the light transmission characteristicof the liquid crystal cell is selectively controlled by applying voltageto the electrode pair, thereby displaying the desired picture image.

The above-structured liquid crystal displays are representative ofportable flat panel displays. Among them, thin film transistor liquidcrystal displays (TFT LCD) with thin film transistors as switchingcircuits have been extensively used.

In such a thin film transistor liquid crystal display, thin filmtransistors are formed on a substrate such that they correspond topixels arranged in a matrix form. The substrate with the thin filmtransistors formed thereon is usually called the “thin film transistorarray substrate.” A pixel electrode is formed at each pixel of the thinfilm transistor array substrate such that it receives picture signalsdepending upon the control of the corresponding thin film transistor.Gate and data lines are formed on the thin film transistor arraysubstrate such that they are connected to the pixel electrodes via thethin film transistors. The data lines cross over the gate lines tothereby define pixels in a matrix form. The gate lines are connected tooutput terminals of gate driving integrated circuits to receive gatesignals and transmit them to the pixel electrodes. The data lines areconnected to output terminals of data driving integrated circuits toreceive picture signals and transmit them to the pixel electrodes.

FIG. 1 illustrates the conceptual structure of a conventional liquidcrystal display. In the drawing, G1 to Gm indicate the gate lines, S1 toSn indicate the data lines, P indicates the pixel electrode, and TFTindicates the thin film transistor.

When the driving voltage of the same polarity is continuously applied tothe liquid crystal cell, the pixel electrode and the counter electrodechange electrochemically due to the saturation of ionic impurities inthe liquid crystal material, and this deteriorates the displaysensitivity and the brightness.

In order to prevent such a defect, the polarity of voltage applied tothe liquid crystal cell is required to be inverted in a cyclic manner,and this driving technique is called the “inversion driving technique”.Such inversion driving techniques include a frame inversion where thepolarity is inverted per a frame, a line inversion where the polarity isinverted per a line, and a dot inversion where the polarity is invertedper a pixel. Among the techniques, the line inversion or the dotinversion is mainly used.

The dot inversion driving technique applies the driving voltages of theopposite polarity to the two pixel electrodes neighboring each other inthe column and row directions. A driving voltage of positive polarity isapplied to one of the neighboring pixel electrodes, and a drivingvoltage of negative polarity is applied to the other pixel electrode.This polarity state is inverted per each frame.

The dot inversion driving techniques has two methods. One is a 1 dotinversion driving where the vertically and horizontally neighboringpixel electrodes bear opposite polarity. The other is a 2-1 dotinversion driving where the horizontally neighboring pixel electrodesbear the opposite polarity but the polarity of the verticallyneighboring pixel electrodes is inverted per two rows. The 2-1 dotinversion driving technique has several advantages over the 1 dotinversion driving technique. Reduced power consumption and no-flickeringat the window screen are examples.

FIG. 2A illustrates the polarity state of pixels in a liquid crystaldisplay where the 2-1 dot inversion driving technique is used. FIG. 2Billustrates the brightness of the pixels shown in FIG. 2A. FIG. 2Cillustrates the voltage storage of the pixels shown in FIG. 2A.

In the 2-1 dot inversion driving technique, voltages of the samepolarity are applied to the pixel electrodes per two pixel rows. Forthis reason, as shown in FIG. 2B, the voltage storage between thevertically neighboring pixel electrodes between up and down varies todeteriorate brightness over the entire screen area and forms dimhorizontal lines.

As shown in FIG. 2B, when the first pixel row #1 and the second pixelrow #2 are charged with the positive (+) polarity, and the positive (+)data is inverted into the negative (−) data at the third pixel row #3,an AC current is generated due to the parasitic capacitance between thepixel electrodes at the second pixel row #2 and the pixel electrodes atthe third pixel row #3. This deteriorates the charge rate of the pixelelectrodes at the second pixel row #2.

Therefore, among the two pixel rows that receive gray scale voltages ofthe same polarity, the brightness at the second pixel row becomes lowerdue to the charge rate deterioration compared to the first pixel row,thereby generating faint difference in brightness per a pixel row, thatis, per a gate line.

Furthermore, when voltage delay occurs due to the slew rate withoutapplying an ideal square wave, the charge rate deteriorates at the firstpixel row. As a result, in the two pixel rows under receiving voltagesof the same polarity, the brightness at the first pixel row is reducedcompared to the second pixel row. Therefore, brightness differenceoccurs even at the pixel rows receiving voltages of the same polarity.Consequently, horizontally extended bands are displayed at the screenwhile deteriorating the display characteristic.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a liquidcrystal display which bears uniform brightness characteristics over theentire screen area.

This and other objects may be achieved by a liquid crystal display withthe following features. The liquid crystal display includes a liquidcrystal display panel having a plurality of gate lines. A plurality ofdata lines cross over the gate lines while being electrically insulatedfrom the gate lines. Pixels are placed at the cross regions of the gateand the data lines in a matrix form. Each pixel has a switching circuitconnected to the gate and the data lines. The polarity of the pixels isinverted per a pixel group of two or more pixel rows. The liquid crystaldisplay further includes a data driving unit, and a scan driving unit.The data driving unit feeds gray scale voltages to the data lines. Thescan driving unit feeds gate voltages of different levels to theneighboring first and second gate lines.

The scan driving unit feeds a first gate voltage of a predeterminedlevel to the first gate line while feeding a second gate voltage ofanother predetermined level to the second gate line. The first gatevoltage is greater than, or lower than the second gate voltage.

In a method of driving the liquid crystal display, data voltages are fedto the pixels such that the polarity of the pixels is inverted per apixel group of two or more pixel rows. Gate voltages of different levelsare fed to the neighboring first and second gate lines.

A first gate voltage of a predetermined level is fed to the first gateline while feeding a second gate voltage of another predetermined levelto the second gate line. The first gate voltage is greater than, orlower than the second gate voltage. The gate voltages fed to the gatelines may bear two or more different values.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention, and many of the attendantadvantages thereof, will be readily apparent as the same becomes betterunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings in which likereference symbols indicate the same or the similar components.

FIG. 1 schematically illustrates the plane structure of a conventionalliquid crystal display panel.

FIG. 2A illustrates the polarity of each pixel of a liquid crystaldisplay where a 2-1 dot inversion driving technique is used.

FIG. 2B illustrates the brightness of each pixel of the liquid crystaldisplay shown in FIG. 2A.

FIGS. 2C illustrates the voltage storage of each pixel of the liquidcrystal display shown in FIG. 2A.

FIG. 3 is a block diagram of a liquid crystal display according to apreferred embodiment of the present invention.

FIG. 4 is a graph illustrating the gate driving voltage characteristicper each pixel for the liquid crystal display shown in FIG. 3.

FIG. 5 is a circuit diagram illustrating the circuit structure of a scandriving unit for the liquid crystal display shown in FIG. 3.

FIG. 6 is an operational timing diagram of the scan driving unit shownin FIG. 5.

FIG. 7 is a graphs illustrating the waveforms of output voltages of thescan driving unit shown in FIG. 5.

FIG. 8 illustrates the driving state of the gate lines for the liquidcrystal display shown in FIG. 3 and the polarity state of the pixelspursuant thereto.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of this invention will be explained with referenceto the accompanying drawings. FIG. 3 schematically illustrates thestructure of a liquid crystal display according to a preferredembodiment of the present invention. As shown in FIG. 3, the liquidcrystal display includes an LCD panel 1, a scan driving unit 2, a datadriving unit 3, a Von, Voff and Vcom generation unit 4, a timing controlunit 5, and a gray scale voltage generation unit 6. Signals are appliedto the LCD panel 1 through the data driving unit 3 and the scan drivingunit 2.

A plurality of gate lines are formed in the LCD panel 1 to transmit gatedriving signals. A plurality of data lines are also formed in the LCDpanel 1 and cross over the gate lines to transmit gray scale voltagescarrying picture signals. A pixel is formed at a region where one dataline crosses over one gate lines. That is, the pixels are arranged in amatrix form.

The data driving unit 3, usually called the “source driving unit”, loadsvoltages to the pixels within the LCD panel 1. Specifically speaking,the data driving unit 3 stores the digital data from the timing controlunit 5 in its shift resister. Upon receipt of signals (LOAD) instructingto load the data onto the LCD panel 1, the data driving unit 3 selectsthe voltages corresponding to the respective data, and transmits theselected voltages to the LCD panel 1.

The scan driving unit 2, usually called the “gate driving unit”,controls the data transmission from the data driving unit 3 to thepixels. Each pixel of the LCD panel 1 becomes an on or off state byturning on or off a thin film transistor (TFT) as a switching unit. TheTFT is turned on or off depending on voltage Von or Voff applied to thegate thereof. The voltages Von and Voff are generated from the Von, Voffand Vcom generation unit 4. The Von, Voff and Vcom generation unit 4generates the Von voltage and the Voff voltage as well as the Vcomvoltage that is a reference value for the difference in the data voltagewithin the TFT.

The timing control unit 5 generates digital signals for driving the datadriving unit 3 and the scan driving unit 2. Specifically, the timingcontrol unit 5 generates signals for the scan driving unit 2 and thedata driving unit 3, for controlling the timing of the data and forcontrolling the clock. The gray scale voltage generation unit 6generates gray scale voltages for the data driving unit 3.

In the above-structured liquid crystal display, the polarity of thepixel electrode is inverted per at least two pixel rows. Furthermore,the horizontally neighboring pixel electrodes at one pixel row bear theopposite polarity. For this purpose, the timing control unit 5 generatesdriving signals for inversion-driving the LCD panel 1, and feeds them tothe data driving unit 3 and the scan driving unit 2. The data drivingunit 3 feeds gray scale voltages of the relevant polarity to the datalines in adaptation to the driving signals (the data signals) from thetiming control unit 5.

In order to prevent deterioration in the charge due to inversion in thepolarity between the vertically neighboring pixels, the scan drivingunit 2 applies gate driving signals Von to the pixels such that they aredifferentiated at the respective pixels. FIG. 4 illustrates the gateline driving voltage characteristic. Specifically, the scan driving unit2 generates gate driving signals of variable value depending on thesignal outputs from the timing control unit 5, and feeds them to thegate lines. For instance, in the 2-1dot inversion driving technique, thepolarity of the pixel electrode is inverted per two pixel rows. When theneighboring pixel electrodes at the same pixel row bear the oppositepolarity, the scan driving unit 2 generates gate driving signalsinverted per a 1H cycle, and feeds them to the gate lines, therebycompensating for difference in the charge made per each line.

FIG. 5 illustrates the structure of the scan driving unit 2 for feedingthe gate driving signals to the LCD panel. As shown in FIG. 5, the scandriving unit 2 includes a signal generation unit 21 for generating aplurality of driving signals upon receipt of gate driving clocks CPV andhorizontal synchronization pulses STV. First and second D-typeflip-flops 22 and 23 are operated upon receiving the driving signalsoutput from the signal generation unit 21 to generate signals that areinverted per a predetermined cycle. An output unit 24 stabilizes thesignals output from the first and second D-type flip-flops 22 and 23.

The signal generation unit 21 includes a transistor T1 that switchesbetween on and off states depending upon the gate driving clocks CPV,and a transistor T2 that switches between on and off states dependingupon the horizontal synchronization pulses STV. Resistors R1 to R4 areconnected to a base terminal and a collector terminal of each transistorT1 or T2.

The clock terminal CLK1 of the first D-type flip-flop 22 is connected tothe collector terminal of the transistor T1, and the input terminal D1is connected to the inversion output terminal Q1. The input terminal D2of the second D-type flip-flop 23 is connected to the output terminal Q1of the first flip-flop 22, and the clock terminal CLK2 is connected tothe gate driving clock CPV. The clear terminals CLR1 and CLR2 and thepre-set terminals PR1 and PR2 of the first and the second D-typeflip-flops 22 and 23, respectively, are connected to the collectorterminal of the transistor T2. The operational characteristics of theD-type flip-flop are listed in Table 1.

TABLE 1 Input Output PR CLR CLK D Q /Q L H X X H L H L X X L H L L X X HH H H ↑ H H L H H ↑ L L H H H L X Q0 /Q0

The timing diagram of the above-structured scan driving unit isillustrated in FIG. 6, and the waveforms of the output voltages areillustrated in FIGS. 7A and 7B.

The transistors T1 and T2 turn on or off depending upon the gate drivingclocks CPV and the horizontal synchronization pulses STV output from thetiming control unit 5. With turning on or off of the transistors T1 andT2, the “L” or “H” level signals are input into the clock terminal CLK1of the first D-type flip-flop 22, and the clear terminals CLR1 and CLR2as well as the preset terminals PR1 and PR2 so that the D-typeflip-flops 22 and 23 start to operate.

As shown in FIG. 6, when the “H” level gate driving clocks CPV and thehorizontal synchronization pulses STV are input into the transistors T1and T2, the transistors T1 and T2 turn on so that the “L” level signalsare input into the clear terminals CLR1 and CLR2 and the presetterminals PR1 and PR2. In accordance with the operation characteristicslisted in Table 1, the output of the first and the second D-typeflip-flops 22 and 23 is maintained to be in the “H” state irrespectiveof the input.

Thereafter, when the “L” level gate driving clocks CPV and thehorizontal synchronization pulses STV are input, the transistors T1 andT2 turn off so that the “H” signals are input into the clock terminalCLK of the first D-type flip-flop 22 as well as into the clear terminalsCLR1 and CLR2 and the preset terminals PR1 and PR2. In accordance withthe operational characteristics listed in Table 1, the first and thesecond D-type flip-flops 22 and 23 output the “H” or “L” level signalsin synchronization with the clock terminals CLK1 and CLK2.

Meanwhile, as the inversion output terminal/Q1 of the first D-typeflip-flop 22 is connected to the input terminal D1 of the first D-typeflip-flop 22, signals having a level opposite to the input signals isoutput from the first D-type flip-flop 22, and input into the secondD-type flip-flop 23. The signals are then output in synchronization withthe gate driving clocks CPV input into the clock terminal CLK2 of thesecond D-type flip-flop 23. Accordingly, as shown in FIG. 6, the signalschanged in the voltage level per 1H cycle in synchronization with thegate driving clocks CPV are output as the gate driving voltage Von. Thewaveform of the output voltages are illustrated in FIG. 7 as (a) and(b).

The circuit for generating the gate driving signals changed in thevoltage level per predetermined cycle (for example, the 1H cycle) is notlimited to the above-described structure, but may be structured invarious manners. Furthermore, instead of the scan driving unit, thetiming control unit may generate the signals changed in the voltagelevel per 1H cycle, and feed them to the required place.

A method for driving the above-structured liquid crystal display will benow explained in detail. The polarity of the respective pixels in theliquid crystal display is the same as that related to the 2-1 inversiondriving technique. Upon receipt of picture signals Vs from a signalsource (not shown), the timing control unit 5 processes the picturesignals into data signals, and transmits the data signals to the datadriving unit 3. Furthermore, the timing control unit 5 generates variouskinds of timing signals required for driving the liquid crystal displaysuch as gate driving clocks CPV and horizontal synchronization pulsesSTV.

The data driving unit 3 applies the data voltages (the gray scalevoltages) to each pixel of the LCD panel 1 depending upon the datasignals from the timing control unit 5. The scan driving unit 2 outputsthe gate voltages as the gate driving signals that turn on the thin filmtransistor of each pixel to apply the data voltages to the pixel.

The gray scale voltages of the same polarity are fed to the respectivepixels per two pixel rows. When the gate line of each pixel row isdriven, gray scale voltages bearing a first polarity and gray scalevoltages bearing a second polarity are alternately fed to the data line.Consequently, gray scale voltages of the opposite polarity are fed tothe neighboring pixels at one pixel row, and voltages of the samepolarity are fed to the pixels per two pixel rows.

For instance, in case gray scale voltages are fed to the data lineswhile driving N numbers of gate lines in a sequential manner, they arefed to the data lines in the polarity order of “+, −, +, −, +, −, . . .” during the operation of the first and the second gate lines, whilebeing fed thereto in the order of“−, +, −, +, −, +, . . . ” during theoperation of the third and fourth gate lines. As a result, the datalines bear the polarity distinction shown in FIG. 2A.

The scan driving unit 2 feeds the gate voltages changed in the voltagelevel per a cycle of 1H to each pixel electrode to charge the pixelelectrode with sufficient voltage. That is, in order to prevent theparasitic capacitance between the vertically neighboring pixelelectrodes from deteriorating the charge at the inversion of the voltagepolarity due to, as shown in FIG. 8, a first gate voltage is fed to thefirst gate line, and a second gate voltage greater than the first gatevoltage is fed to the second gate line. Furthermore, a first gatevoltage is fed to the third gate line where the polarity of the grayscale voltages fed to the respective pixels varies, and a second gatevoltage is fed to the fourth gate line.

As the gate lines (i.e., the second gate line, the fourth gate line,etc.) between the pixel rows of the opposite polarity receive greatergate voltage compared to the gate lines (i.e., the first gate line, thethird gate line, etc.) between the pixel rows of the same polarity,deterioration in the voltage charge at the polarity inversion due to theparasitic capacitance between the vertically neighboring pixelelectrodes can be prevented.

Meanwhile, if the voltage signal is delayed due to the slew rate withoutapplying the gate voltage of an ideal square wave to the gate lines, thefirst gate line receives a gate voltage greater than the voltage of thesecond gate line. That is, a second gate voltage is fed to the firstgate line, and a first gate voltage is fed to the second gate line. Inthis way, deterioration in the voltage charge due to the delay in thevoltage signal is prevented. Accordingly, the same voltage charge ismade at each pixel row per a gate line so that the entire screenbrightness can be kept uniform. The gate driving signals (i.e., the gatevoltages) may vary appropriately.

The above-described effects may be made also with respect to a 3-1 or4-1 dot inversion type liquid crystal display where the inter-pixelpolarity is inverted per three or four pixel rows. Furthermore, the gatevoltage may bear two or more values.

As described above, in the inventive liquid crystal display where theinter-pixel polarity is inverted per two or more pixel rows, thebrightness difference in the pixels due to the deteriorated voltagecharge is compensated to keep brightness characteristic over the entirescreen area uniform, while improving the display characteristic.

While the present invention has been described in detail with referenceto the preferred embodiments, those skilled in the art will appreciatethat various modifications and substitutions can be made thereto withoutdeparting from the spirit and scope of the present invention as setforth in the appended claims.

1. A liquid crystal display (LCD), comprising: a scan driving unitgenerating a first gate voltage and a second gate voltage different fromthe first gate voltage and comprising a first D-type flip-flop and asecond D-type flip-flop; a data driving unit generating gray scalevoltages; a plurality of gate lines transmitting the first gate voltageand the second gate voltage from the scan driving unit; a plurality ofdata lines crossing the gate lines and transmitting the gray scalevoltages from the data driving unit; a plurality of pixels arranged in amatrix form and divided into a plurality of pixel row groups, each pixelrow group comprising a first pixel row and a second pixel rowneighboring the first pixel row, polarities of the pixels being invertedper the pixel row group; a plurality of pixel electrodes provided to thepixels, respectively; and a plurality of thin film transistors (TFTs)provided to the pixels, respectively, each TFT comprising a gateelectrode connected to the corresponding gate line, a source electrodeconnected to the corresponding data line, and drain electrode connectedto the corresponding pixel electrode, wherein the first gate voltage istransferred to the gate electrode of the first pixel row and the secondgate voltage is transferred to the gate electrode of the second pixelrow.
 2. The liquid crystal display of claim 1, wherein the first gatevoltage is higher than the second gate voltage.
 3. A method of driving aliquid crystal display (LCD), the LCD comprising a scan driving unitcomprising a first D-type flip-flop and a second D-type flip-flop, adata driving unit, a plurality of gate lines, a plurality of data lines,a plurality of pixels arranged in a matrix and divided into a pluralityof pixel row groups, each pixel row group comprising a first pixel rowand a second pixel row neighboring the first pixel row, a plurality ofthin film transistors (TFTs) provided to the pixels, respectively, eachTFT comprising a gate electrode connected to the corresponding gate lineand a source electrode connected to the corresponding data line, themethod comprising steps of: inverting polarities of the pixels per thepixel row group; and feeding different gate voltages generated using thefirst D-type flip-flop and the second D-type flip-flop to the gateelectrode of the first pixel row and the gate electrode of the secondpixel row, respectively.
 4. The method of claim 3, wherein the step offeeding the gate voltage comprises steps of: feeding a first gatevoltage to the first gate line; feeding a second gate voltage to thesecond gate line, the first gate voltage being higher than the secondgate voltage.
 5. A method for driving a liquid crystal display(LCD), theLCD comprising a scan driving unit comprising a first D-type flip-flopand a second D-type flip-flop, a data driving unit, a plurality of gatelines, a plurality of data lines, a plurality of pixels divided into aplurality of pixel row groups, each pixel row group comprising a firstpixel row and a second pixel row neighboring the first pixel row, and aplurality of thin film transistors (TFTs) provided corresponding to thepixels, each TFT comprising a gate electrode connected to thecorresponding gate line and a source electrode connected to thecorresponding data line, the method comprising a step of feedingdifferent gate voltages generated using the first D-type flip-flop andthe second D-type flip-flop to the gate electrode of the first pixel rowand the gate electrode of the second pixel row, respectively.